ASHS 2024 Conference

Somatic mutations in apple commonly develop into viable bud sports for cultivation. When an apple bud sport has a desirable attribute such as improved color, size, shape, flavor, firmness, sweetness, or harvest timing, it has potential to be introduced as a new cultivar that growers utilize, and consumers enjoy. The genetic mutations and related mechanisms associated with early or delayed maturation (respectively resulting in early or late harvest date) in apple sports are not known despite their value to the industry. By acquiring knowledge about these genetic mutations and their respective molecular mechanisms, breeders can identify markers to conduct more informed crosses to select for early or late maturing apple lines. Apple lines with differing developmental rates resulting in differing ‘designed’ harvest windows would optimize the deployment of field laborers for the apple harvest and improve use of cultural practices to influence harvest time. Comparison of physiology-based markers of fruit developmental stages for apple sports with different maturation dates has been done, but, to the author’s knowledge, not on trees for which crop load has been appropriately adjusted to eliminate its effect on developmental rate. In this study, crop load was regulated on 6 different apple cultivars according to tree trunk cross-sectional area. The sports selected were the ‘Gala’ sport ‘Autumn Gala’ also known as ‘Harry Black’, the early maturing ‘Fuji’ sport ‘September Wonder Fuji’, and the early maturing ‘Cripps Pink’ (‘Pink Lady®’) sport ‘Maslin Cripps Pink’. These three sports were compared to their progenitor/standard harvest time cultivar. Apple fruits were measured on each of five trees per cultivar, and volumetric growth rate was analyzed. We found that in each comparison, fruit growth rate of the early variant was significantly greater early in fruit development, roughly coinciding with the expected conclusion of the cell division phase based on phenology. Consequently, a narrower window of genetic events responsible for the early or late harvest timing phenotype has been captured. Our work confines the search for genetic events responsible for the harvest date mutations to the early developmental stages for apple fruit. We propose that harvest date in these apple bud sports is physiologically predetermined from the very early stages of fruit development and very likely not a function of strictly ripening-related processes.